A team of scientists have used
Chandra to observe a halo around the X-ray source
Cygnus X-3. The halo (beyond the yellow ring in
the center) is due to scattering by interstellar dust
grains along the line of sight to the source. The sharp
horizontal line is an instrumental effect.

The X-ray emission from Cygnus X-3 is due to matter
falling from a normal star onto a nearby neutron star
or black hole. Its X-ray emission varies regularly with
a 4.8 hour period, as the compact star circles a
companion star.

The radiation from the halo is delayed and smeared
out, so the variations are damped. By observing the
delays and smearing at different parts of the halo, the
distance to the X-ray source is found to be 30,000
light years.